How common should CTDs be?

They shouldn't be that frequent, but with Oblivion, one can never know. But you don't have to worry about any Conflicts, so that would help lessen crashes.

As long as your computer (CPU, chipset, memory, video card) is 100% stable then few if any. I sometimes get a crash on close which is a little more common.

I always crash on close. It doesn't matter though, it makes exiting the game quicker